A straight line to proper shaft and belt alignment

Misalignment of shafts and belts can become a highly vexing problem in plants across industries, and accurate diagnosis and timely remedial action can make all the difference. Properly aligned shafts and belts in rotating machinery can contribute significantly to sustained equipment uptime and productivity.

10/15/2008


Misalignment of shafts and belts can become a highly vexing problem in plants across industries, and accurate diagnosis and timely remedial action can make all the difference. Properly aligned shafts and belts in rotating machinery can contribute significantly to sustained equipment uptime and productivity.

Misaligned shafts can lead to premature bearing or coupling failure, shaft fatigue, damage to seals and increased vibration levels, friction, overheating and energy consumption. Misalignment in belt-driven machinery can increase wear on pulleys and belts, noise and vibration. All of these symptoms ultimately can cause machinery breakdowns, unplanned downtime, higher maintenance costs and loss of production.

The traditional toolbox of technologies to align shafts and belts is equipped with various, relatively low-tech methods. For shaft alignment, these include a straight edge and feeler gauge or dial indicator. For belt alignment, practices shift to visual judgment %%MDASSML%% eye-balling %%MDASSML%% with a straight edge and/or a string. The universal disadvantage with all is their lack of precision and accuracy. However, a bright spot has emerged. Laser alignment technology facilitates accurate alignment, and it performs light years ahead of conventional methods.

Turning to shaft alignment

When an electric motor, turbine or other driver is coupled to a pump, generator or other equipment, the shafts of the two must be properly aligned. Otherwise, parallel or angular misalignment will cause stress on the shafts and adversely affect machinery health and operation. The overall objective of shaft alignment is to increase the operating lifespan of rotating machinery. In particular, accurately aligned machinery reduces a variety of unpleasant problems, not the least of which are excessive axial and radial forces on the bearings. It also minimizes the amount of shaft bending from the point of power transmission in the coupling to the coupling end-bearing and wear in the coupling components, among other problem conditions. In turn, accurate shaft alignment can help increase bearing life, reduce stress on couplings and minimize risk of overheating and breakage. It inhibits wear on seals to help prevent contamination and lubricant leakage, reduces friction and energy consumption and minimizes noise and vibration.

The demands for accuracy, simplicity and reliability in methods used to detect and remedy shaft misalignment have paved the way for the introduction of highly precise laser alignment tools.

One such system employs two measuring units (both provided with a laser diode), positioning detectors and a display unit. The measuring units can be fitted with magnetic brackets for attachment to the shafts, or they can be attached with chains. Both units emit a laser line, which is projected on a position sensor detector and a line scale on the other unit.

During the measuring cycle, the shafts are rotated and readings are taken at the 9-, 3- and 12-o’clock positions. Any relative movement of the laser lines during this rotation indicates some sort of misalignment. The logic circuitry within the tool translates this movement to misalignment figures and prompts corrective action. All calculations are handled in real time, allowing the progress of the alignment to be followed as the process is performed.

Variations driven by applications have broadened the technology’s horizons. Intrinsically-safe shaft alignment tools, certified according to ATEX standards, have been developed for use in potentially explosive and otherwise hazardous areas, such as those encountered in the petrochemical, gas and pharmaceutical industries.

Pinpointing belt alignment

The importance of proper alignment extends to belts in helping prevent machinery breakdowns. Belt and pulley alignment go hand-in-hand because proper belt alignment hinges on the correct alignment of the pulleys on which the belt runs.

The objective is to align belts in a way that results in the least wear on them and the lowest energy loss for the machine or driver unit. When belts are misaligned, risks rise for increased wear on belts, pulleys, axles, shafts and bearings. Friction increases, as does energy consumption, noise and vibration. All of this can lead to decreased bearing life and belt failures. Unless misalignment is corrected, a new belt will last no longer than the one it replaces.

Various types of belt misalignment can occur, and it is not unusual for a combination of different misalignments to be encountered simultaneously. Users may experience vertical angle or twisted misalignment when one of the pulleys has an angular error from the vertical angle plane. Horizontal angle misalignment occurs when the shafts of the driver and the driven unit are not positioned parallel to each other, and parallel misalignment or offset is caused by either the incorrect positioning of the motor along its shaft axis or by incorrect positioning of the pulleys on their respective shaft. It is important that an alignment tool both diagnoses and informs to accommodate resolution of these multiple conditions.

Belt alignment can be accomplished either by traditional or laser methods. Much of the accuracy of the low-tech methods depends on visual judgment alone or in combination with a straight edge and/or length of string. The major disadvantage, as with lower-tech shaft alignment methods, is the lack of accuracy. Belt manufacturers usually recommend a maximum misalignment of 0.5 degrees, or even 0.25 degrees, which is difficult %%MDASSML%% if not impossible %%MDASSML%% to achieve by using visual judgment.

The fact is that low-tech methods involve repeated trial and error, naked-eye approximation and rough estimation, leaving high margins of error unacceptable for precision belt alignment. Laser methods provide a marked improvement by enabling alignment to be adjusted with far more speed and accuracy.

Laser systems can be categorized according to the way the devices can be attached to the pulley and the way they align. In general, the systems are paired: one unit aligns the face of the pulleys while the other aligns in the grooves of the pulleys.

Those using the face or side of the pulley as a reference for aligning the pulleys and belts can be applied for belt types other than V-belts, such as timing belts. The most notable disadvantage of this technique is that only the face of the pulley is used as a reference. Only the faces of the pulleys are aligned with each other. This can result in varying degrees of accuracy when the pulleys are different thicknesses, brands or types, or when the pulley faces are not well-finished.

In contrast, laser devices align the grooves of the pulleys in which the belt runs, substantially increasing accuracy regardless of pulley thickness or type. A system equipped with specially designed V-guides that allow the alignment tool’s two components to attach magnetically into the grooves of the pulley demonstrates this.

The laser unit emits a laser line projected onto a three-dimensional target area on the opposite receiver unit. Depending on the projected laser pattern, the user can determine the type of misalignment %%MDASSML%% vertical or horizontal angle, parallel or all three %%MDASSML%% and how to correct it. Belt alignment can be achieved by adjusting the machines until the laser line coincides with the reference lines on the receiver unit. Side adapters allow this technology to accommodate chain sprockets and timing and ribbed belt alignment.

Low-tech methods of alignment involve repeated trial and error, naked-eye approximation and rough estimation, leaving high margins of error unacceptable for precision belt alignment.

Accurate shaft alignment can help increase bearing life; reduce stress on couplings and minimize risk of overheating and breakage.


Author Information
Paul Michalicka is North American area sales manager for SKF Maintenance Products. He can be reached at (416) 299-2894 or paul.michalicka@skf.com .




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